Last week, I highlighted that the Average Directional Index (ADX) had just hit a level that usually precedes a big move in the S&P 500. Now the transports—a critical leading indicator for the broad market—may be cluing us in to what that move will be.

The Dow Jones Transportation Average, the most widely recognized gauge of the US transportation sector, just fell below declining key moving averages for the first time since 2012.

After breaking out above 8,600 in October, the transports have tested this level eight times. Investors may not need to pack it up just yet, but a close below this level could indicate that the market is about to take a turn for the worse.